Micromachined comb drive for quantitative nanoindentation

ABSTRACT

A microelectromechanical (MEMS) nanoindenter transducer including a body, a probe moveable relative to the body, an indenter tip coupled to an end of the moveable probe, the indenter tip moveable together with the probe, and a micromachined comb drive. The micromachined comb drive includes an electrostatic actuator capacitor comprising a plurality of comb capacitors configured to drive the probe, together with the indenter tip, along a displacement axis, including in an indentation direction, upon application of a bias voltage to the actuation capacitor. The micromachined comb drive further includes a plurality of sensing capacitors forming a differential capacitive displacement sensor, each sensing capacitor comprising a plurality of comb capacitors and each configured to provide capacitance levels which, together, are representative of a position of the probe, wherein each of the comb capacitors of the actuator capacitor and the sensing capacitors includes a fixed electrode comb coupled to the body and a moveable electrode comb coupled to the probe.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Utility Patent Application claims benefit of U.S. ProvisionalApplication 61/077,984, filed Jul. 3, 2008, and which is incorporatedherein by reference.

BACKGROUND

Nanoindentation (see References 1 and 2) is a method to quantitativelymeasure a sample's mechanical properties, such as elastic modulus andhardness, for example, using a small force and a high resolutiondisplacement sensor. Typically, a force employed in nanoindentation isless than 10 mN, with a typical displacement range being smaller than 10μm, and with a noise level typically being better than 1 nm rms.

In nanoindentation, a nanoindenter capable of determining the loadingforce and displacement is used. The force and displacement data are usedto determine a sample's mechanical properties (see Reference 3). Forthis sample property estimation, a nanoindenter has to be integratedwith a characterized tip which has known geometry and known mechanicalproperties.

One of the emerging nanoindentation applications is quantitativetransmission electron microscopy (TEM) in-situ mechanical testing (seeReferences 4, 5, 6, and 7). This testing method enables monitoring ofthe deformation of a sample in real time while measuring thequantitative mechanical data. Due to the limited available space in aTEM holder, however, there is a demand for a miniature transducer.

One of the key components in nanoindentation instrumentation is atransducer which converts an electrical input into a mechanical forceand a mechanical displacement into an electrical signal. A well designednanoindenter transducer can improve many aspects of the nanoindenterperformance such as increasing the range of forces, including increasingthe maximum force, improving force resolution and system bandwidth, andreducing system noise. The present disclosure describes embodiments of amicro-electro-mechanical system (MEMS) transducer for nanoindentationapplications. According to embodiments described herein, the MEMStransducer employs a micromachined comb drive for actuation and sensing.Such a comb drive is advantageous because it provides a largeroverlapping area of electrodes of actuation and sensing capacitorswithin a limited small space relative to conventional transducers, whichincreases an available maximum indentation force and improves thesensitivity of displacement sensing.

Limitations of Conventional Technology with Respect to Actuation

MEMS transducers have been used for nanomechanical test applicationssuch as fracture testing (see references 8 and 9), tensile testing (seeReferences 10, 11, 12 and 13), and indentation (see Reference 5 and 15).However, among known MEMS based nanomechanical testers only one is knownto have been used for nanoindentation. This known nanoindenter uses onlytwo plates for capacitive displacement sensing and the indentation forceon the sample is applied using piezo actuation and spring reaction. Thepenetration depth is estimated by subtracting the actuation distancefrom the indenter displacement.

However, the estimated penetration depth from this operation issusceptible to error from false piezo distance estimation which commonlyhappens due to undesirable piezo characteristics, such as creep,hysteresis in loading and unloading, and the nonlinearity of the piezodisplacement, for example. Since nanoindentation uses a smallpenetration depth, a small error in piezo displacement estimation cancause a relatively large error in sample property estimation.

For this reason, an integrated actuator which enables direct penetrationdepth measurement by making the sensed displacement the same as thepenetration depth is highly desirable for accurate nanoindentationexperimentation.

Limitations of Conventional Technology with Respect to Sensing

Some conventional MEMS based nanomechanical testers utilize capacitancechange for displacement sensing (see References 5, 6, 10, 11, 17, and18). However, most conventional MEMS-based mechanical testers employ asensing capacitor having only one pair of plates or electrodes fordisplacement measurement. Displacement measurement using a sensingcapacitor having only a single pair of electrodes is not desirable fornanomechanical testing because such a measurement scheme is subject toerrors in the displacement sensing due to environmental changes. Such adisplacement sensing scheme also has a relatively large nonlinearitywhich increases as a gap between the pair of electrodes decreases.

Another way to utilize the capacitive sensing for displacementmeasurement is to employ differential capacitive sensing. Onedifferential capacitive sensor utilizes three electrodes. One of theelectrodes is a moveable center electrode. The other two counterelectrodes are fixed and placed in opposite directions from the movablecenter electrode. A displacement sensing scheme employing a differentialcapacitive sensor has less undesirable effects from environmental changeand parasitic capacitance. However, the capacitance change caused by anundesirable source affects each of the two capacitors equally so thatthe undesirable capacitance change is cancelled out by thedifferentiation.

One MEMS based nanomechanical tester (see Reference 10) employsdifferential capacitance sensing using a surface micromachined combdrive sensor. In general, as compared to bulk micromachined comb drives,the electrodes of the sensing capacitors of surface micromachined combdrives have less overlapping area due to a limited plate height, whichlowers the displacement sensitivity of the transducer.

By arranging the comb drives in orthogonal directions, a comb drivesensor can have multidimensional sensing capabilities. One example of acomb drive sensor integrated with a MEMS mechanical tester (seeReferences 11, 17, and 18) realizes 2-axis force sensing capabilitieswith orthogonal direction comb arrays. For this multi-axis displacementsensing, each comb drive is used independently for one axis displacementsensing.

However, such a multi-axis displacement sensing scheme requiresadditional comb drives which requires a larger area to implement Thelarger area restricts the applications in which the comb drivetransducer can be used, such as in-situ TEM applications which have verysmall size requirements.

Limitations of Conventional Technology with Respect to Spring Design

In order for nanomechanical testers to provide accurate mechanicaltesting results, movement of the movable electrode or probe should berestricted to the testing direction. For nanoindentation, the motionshould be perpendicular to the sample surface and, although the indenterexperiences a reaction from the sample stiffness, should be maintainedduring the indentation experiment. To maintain the mechanical testingdirection, the transducer springs should be designed to have a soft orflexible characteristic to movement in the testing direction and a stiffor non-flexible characteristic to movement in other directions.

By restricting movement of the electrode or probe to the testingdirection, measurement error caused by force components which areirrelevant to the testing can be minimized. Among conventionalmechanical testers, one tribometer (see Reference 11) has springsspecially designed for its testing purpose. The springs of thistribometer are designed to have soft lateral or rotational stiffness andlarge indentation direction stiffness for small friction measurement.However, such stiffness characteristics are opposite to characteristicswhich are desirable for nanoindentation. As described above, atransducer for nanoindentation application should have soft indentationdirection stiffness and large lateral stiffness in order to penetratethe sample perpendicular to its surface plane.

In addition to the stiffness related quasi-static characteristics, thespring design has an effect on the dynamic mechanical analysis. Dynamicmechanical analysis (DMA) measures the frequency characteristics of asample, such as storage and loss moduli, for example, by measuring andthen converting the amplitude and phase response into the mechanicalproperties of the sample. Dynamic mechanical testing has the highestsensitivity to a sample's reactive force when operated at its resonancefrequency.

In order to obtain valid results from dynamic analysis, the dynamic modeshape at the resonance frequency should have a motion in the testingdirection. To prevent coupling with other dynamic modes at the resonancefrequency, the second natural frequency should be separated from theresonance frequency. This natural frequency separation decouples thefirst and the second modes in dynamic operation and improves dynamicmechanical analysis test results.

Dynamic mechanical analysis is based on a single-degree-of-freedomassumption and, to hold such an assumption, complete separation of thesecond mode from the first mode is required. When the second mode iscoupled with the first mode, the frequency response around the resonancefrequency does not match with the single-degree-of-freedomsecond-order-system response and results in errors in the sample'sfrequency characteristics. This requirement must be considered whendesigning springs for nanomechanical testers.

Atomic force microscope (AFM) cantilevers are designed to have desireddynamic characteristics suitable for topography measurement, but aredifficult to use for nanoindentation applications due to tiltingcharacteristics of the tip during indentation.

Limitation of Conventional Technology with Respect to Indenter TipWiring

In some nanoindentation applications, a conductive tip is used which iswired for purposes of electrical measurement or discharging. When anindenter tip is wired, it can be used for in-situ electrical measurementduring the nanoindentation to find the correlation between themechanical and electrical data (see Reference 16). In addition, a wiredconductive tip is used for in-situ electron microscopy nanoindentation(see Reference 4) to discharge the electrons and remove an attractioncaused by the accumulation of electrons. Electrically isolating theconductive tip from the other electrode is difficult for a MEMS devicebecause of its small size and electrical layout limitations. Theindenter tip of one known MEMS nanoindenter (see Reference 5) isconnected to one of the sensing capacitor plates which may causeelectrical drift and an increase in noise. Complete isolation of the tipis desirable to prevent unwanted effects caused by electrons in electronmicroscopy measurement.

Limitations of Conventional Technology with Respect to TransducerPackaging

It is desirable for a MEMS nanomechanical tester to be packaged toprotect the tester from contamination and electrically shield thetransducer. Since a MEMS transducer has many small features which canmalfunction as a result of contamination, protection from contaminationis important to prolong the transducer's life time. Conductive packagingmaterials can be used to electrically shield the transducer. MostMEMS-based nanomechanical testers are not commercialized, and thus therehas been little need to package the transducers. One knownnanomechanical tester, a MEMS nanoindenter, is partially covered, buthas springs and a circular hole designed for tip mounting which areexposed. This exposed area can be contaminated and can also accumulatethe electrons when used in electron microscopy applications.

Limitation of Conventional Technology with Respect to Crash Protection

Due to the small gap distances between the capacitor electrodes in acomb drive, the electrodes can easily contact one another throughimproper operation or mishandling, particularly when a comb drive isused for nanomechanical testing where the comb drive can experienceunstable operation. Even minor damage to the electrodes can effectivelyrender the nanomechanical testing device useless as any damage to thecomb drive destroys the calibration of the testing device so thatmeasurement data cannot be properly converted into a sample's mechanicalproperty properly due to incorrect transducer constants. Such electrodecontact should be prevented to protect the transducer and the controllerelectronics from permanent damage and it can be prevented bymechanically limiting the movable electrode to motion within a saferange. Such a safety feature is not known to be used by any knownMEMS-based mechanical testers.

Limitation of Conventional Technology with Respect to Indenter TipMounting

Measured indentation data comprise a loading and an unloading curvewhich can be converted into sample's mechanical properties. For thisconversion, it is advantageous to employ an indenter tip with definedgeometry. However, mounting an indenter tip on a small device, such as aMEMS device, is difficult due to the small size of the MEMS device andthe indenter tip. In addition to the small size, the fragility of theMEMS material also makes it difficult. Some conventional comb drives canapply a force to a sample (see References 17-19), but the measuredreaction of the sample to the force cannot be converted into mechanicalproperties (e.g. elastic modulus and hardness) because the forcemeasurement is not performed with an indenter tip having a definedgeometry.

Mounting of an indenter tip is one of the main challenges to utilizing aMEMS device as a nanoindenter. One known MEMS nanoindenter includes acircular, deep hole on the transducer for tip mounting. However, thegeometry of this hole is not well optimized to align and permanentlyattach an indenter tip onto the transducer. The tip-transducer contactarea is just a 0.2 mm radius circular face, which might not be largeenough for proper alignment of the tip

REFERENCES

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SUMMARY

One embodiment provides a microelectromechanical (MEMS) nanoindentertransducer including a body, a probe moveable relative to the body, anindenter tip coupled to an end of the moveable probe, the indenter tipmoveable with the probe, and a micromachined comb drive. Themicromachined comb drive includes an electrostatic actuator capacitorcomprising a plurality of comb capacitors configured to drive the probe,together with the indenter tip, along a displacement axis, including inan indentation direction, upon application of a bias voltage to theactuation capacitor. The micromachined comb drive further includes aplurality of sensing capacitors forming a differential capacitivedisplacement sensor, each sensing capacitor comprising a plurality ofcomb capacitors and each configured to provide capacitance levels which,together, are representative of a position of the probe, wherein each ofthe comb capacitors of the actuator capacitor and the sensing capacitorsincludes a fixed electrode comb coupled to the body and a moveableelectrode comb coupled to the probe.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a nanoindentation test system employing aMEMS nanoindenter transducer according to one embodiment.

FIG. 2 is a 3D image of MEMS nanoindenter transducer according to oneembodiment.

FIG. 3 is an exploded view of the MEMS nanoindenter transducer of FIG.2.

FIG. 4 illustrates two set of electrostatic actuator combs according toone embodiment.

FIG. 5 is a microscope image of an actuation capacitor includingmicromachined electrostatic actuator comb capacitors according to oneembodiment.

FIG. 6 illustrates a differential capacitance sensing scheme for sensingcapacitors according to one embodiment.

FIG. 7 is a microscope image of sensing capacitors includingmicromachined comb capacitors according to one embodiment.

FIG. 8 is a diagram illustrating a spring according to one embodiment.

FIG. 9 is a microscope image of a crash protector fabricated on a MEMSnanoindenter transducer, according to one embodiment.

FIG. 10 illustrates graphs showing measured amplitude and phase around aresonance frequency, according to one embodiment.

FIG. 11 is a mode shape at a resonance frequency of a moveable probeobtained from a finite element analysis, according to one embodiment.

FIG. 12 is a microscope image of a micromachined indenter tip mountingtrench on a moveable probe, according to one embodiment.

FIG. 13 is a flow diagram generally illustrating a process forfabrication of a MEMS nanoindenter transducer, according to oneembodiment.

FIG. 14 is a microscope image of a micromachined comb drive according toone embodiment.

FIG. 15 illustrates a top cover for a MEMS nanoindenter transducer,according to one embodiment.

FIG. 16 illustrates a bottom cover for a MEMS nanoindenter transducer,according to one embodiment.

FIG. 17 is an image of a packaged MEMS nanoindenter transducer,according to one embodiment.

FIG. 18 illustrates a load-displacement curve for a polycarbonate sampleand a corresponding sample image, according to one embodiment.

FIG. 19 illustrates a load-displacement curve for a gold sample and acorresponding sample image, according to one embodiment.

FIG. 20 shows scanned topography images on a PMMA sample, according toone embodiment.

FIG. 21 shows modulus mapping related images of a scan area, accordingto one embodiment.

DETAILED DESCRIPTION

In the following Detailed Description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. In this regard, directional terminology, such as “top,”“bottom,” “front,” “back,” “leading,” “trailing,” etc., is used withreference to the orientation of the Figure(s) being described. Becausecomponents of embodiments of the present invention can be positioned ina number of different orientations, the directional terminology is usedfor purposes of illustration and is in no way limiting. It is to beunderstood that other embodiments may be utilized and structural orlogical changes may be made without departing from the scope of thepresent invention. The following detailed description, therefore, is notto be taken in a limiting sense, and the scope of the present inventionis defined by the appended claims.

According to embodiments described herein, a micromachined comb drive isprovided for performing nanoindentation tests to determine surfaceproperties of materials. According to one embodiment, the micromachinedcomb drive includes an actuation comb configured as an electrostaticactuator for actuation of a moveable probe including an indenter tip andfour sensing combs configured as displacement sensors to providedisplacement sensing in two orthogonal directions as well as angularrotation.

FIG. 1 is a block diagram generally illustrating one embodiment of ananomechanical test system 30 employing a MEMS nanoindenter transducer100 according to the present disclosure. In addition to MEMSnanoindenter transducer 100, which includes an indenter tip 205, system30 includes a platform 34 configured to hold a test sample 36 having asurface 38 to be tested via nanoindentation, and a controller 40 incommunication with a computer 42 via an interface 44. Test system 30 isat least suitable for in-situ sample testing.

According to one embodiment, MEMS nanoindenter transducer 100 isconfigured to provide to a detection circuit 60 capacitive signals 50which are representative of a displacement of indenter tip 205 in avertical direction (z-dimension), in orthogonal horizontal directions(x- and y-dimensions), and of rotational movement relative to platform34. According to one embodiment, detection circuit 60 convertscapacitive signals 50 to voltage signal 51. According to one embodiment,controller 40 converts voltage signal 51 to digital signals and providesthe digital signals to computer 42 via interface 44. According to oneembodiment, based on these digital signals, an application module 46(e.g. software) provides a digital actuation signal to controller 40which, in-turn, converts the digital actuation signal to an actuationvoltage signal 52 which is provided to micromachined comb drive 100 soas to actuate or displace indenter tip 205 a desired distance along thez-axis relative to platform 34.

According to one embodiment, controller 40, via application module 46 ofcomputer 42, is configured to control movement of indenter tip 205relative to platform 34 and to provide to computer 42 via interface 44 asignal representative of a displacement of indenter tip 205 from aninitial reference point. According to one embodiment, controller 40 isconfigured to measure and adjust the actuation force.

According to one embodiment, application module 46 comprisesinstructions stored in a memory system 47 that are accessible andexecutable by a processor 48. Memory system 47 may comprise any numberof types of volatile and non-volatile storage devices such as RAM, harddisk drives, CD-ROM drives, and DVD drives. In other embodiments,application module 46 may comprise any combination of hardware,firmware, and software components configured to perform at least thefunctions described herein.

According to one embodiment, nanomechanical test system 30 furtherincludes an imaging device 70 which provides viewing of surface 38 oftest sample 36. According to one embodiment, imaging device 70 comprisesan instrument/device capable of recording or determining the profile orcontour of a test region such as, for example, an optical microscope, aprofilometer, a scanning probe microscope (SPM), or an atomic forcemicroscope (AFM), which is configured to provide images of surface 38 ofsample 36.

Examples of systems similar to test apparatus 30 and suitable to beconfigured for use with the micromachined comb drive and indenter tipaccording to the present disclosure are described by U.S. Pat. Nos.5,553,486 and 5,869,751, both of which are assigned to the same assigneeas the present disclosure and incorporated herein by reference. Anothertest system suitable to be configured for use with the micromachinedcomb drive and indenter tip according to the present disclosure iscommercially available under the tradename TriboIndenter from Hysitron,Incorporated, of Minneapolis, Minn., USA.

FIG. 2 is a perspective view illustrating MEMS nanoindenter transducer100, according to one embodiment of the present disclosure. It is notedthat FIG. 2 illustrates MEMS nanoindenter transducer 100 prior tomounting of indenter tip, which is described in greater detail below(see indenter tip 205 of FIG. 1). According to one embodiment, MEMSnanoindenter transducer 100 includes a body 102 and a moveable probe 104which is coupled to body 102 via springs 106, 108, 110, and 112 in afashion such that moveable probe 104 is displaceable substantially alonga displacement axis 114, including in an indentation direction 116 (e.g.z-dimension with respect to FIG. 1). MEMS nanoindenter transducer 100further includes a micromachined comb drive 119 which includes anactuation capacitor 120 and four sensing capacitors 130, 132, 134, and136, with each of the capacitors comprising a plurality of comb-typecapacitors, which will be described in greater detail below.

As will also be described in greater detail below, MEMS nanoindentertransducer 100 further includes four crash protectors 160, 162, 164, and166. According to one embodiment, as illustrated by FIG. 2, crashprotectors 160 and 162 and crash protectors 164 and 166 are positionedproximate to opposite ends of moveable probe 104 and are configured torestrict displacement of moveable probe 104 to prevent damage to thecomb-type capacitors of actuation capacitor 120 and sensing capacitors130, 132, 134, and 136.

As illustrated, MEMS nanoindenter transducer 100 has a length (L), awidth (W), and a thickness (T). According to one embodiment, MEMSnanoindenter transducer 100 has a length (L) of 5.7 mm, a width (W) of2.8 mm, and a thickness (T) of 0.35 mm. According to one embodiment, dueto space restrictions of some nanoindentation applications, such asquantitative in-situ TEM nanomechanical testing, for example, thecritical dimensions are a thickness (T) of 0.35 mm and a width (W) of2.8 mm. In some applications, such as with Tecnai® G² TEM type holders,for example, the maximum allowable thickness (T) and width (W) to mounta nanoindenter are 2 mm and 4 mm, respectively.

FIG. 3 is an exploded view of MEMS nanoindenter transducer 100 of FIG.1, according to one embodiment, including indenter tip 205. According tothe embodiment of FIG. 2, body 102 of MEMS nanoindenter transducer 100includes four layers: a metal layer 201, a device layer 202, an oxidelayer 203, and a substrate layer 204. Metal layer 201 is deposited ondevice layer 202 and is employed for making electrical connections withan electrical circuit board (not shown). Actuation capacitor 120,sensing capacitors 130, 132, 134, and 136, springs 106, 108, 110, and112, and crash protectors 160, 162, 164, and 166 are fabricated ondevice layer 202. Moveable probe 104 is formed from device, oxide, andsubstrate layers 202, 203, and 204.

According to one embodiment, MEMS nanoindenter transducer 100 ismicromachined from a silicon-on-insulator (SOI) wafer. According to oneembodiment, in order to achieve a high electrical conductivity, heavilyboron doped p-type silicon wafers were used for the device and substratelayers. According to one embodiment, a resistivity of the wafer was0.005-0.02 ohm-cm. According to one embodiment, actuation capacitor 120and sensing capacitors 130, 132, 134, and 136 are fabricated using deepreactive ion etching (DRIE) techniques.

According to one embodiment, with reference to FIG. 4 below, in order toadjust an overlapping area between plates or electrodes of the comb-typecapacitor of actuation capacitor 120 and sensing capacitors 130, 132,134, and 136, a thickness of device layer 202 may be adjusted. Forexample, to increase the overlapping area, the thickness of device layer202 may be increased. However, DRIE capabilities must also be consideredwhen determining the thickness of device layer 202.

According to one embodiment, device layer 202 includes 5 μm features.According to one embodiment, an aspect ratio of the 5 μm features to thethickness is 10:1. Such dimensions can be DRIE etched without largeerror. According to one embodiment, the plates or electrodes of theactuation and sensing comb capacitors of actuation capacitor 120 andsensing capacitors 130, 132, 134, and 136 are electrically isolated bydeep trenches formed so as to penetrate through the device layer 202.

Oxide layer 203 insulates device layer 202 and substrate layer 204.According to one embodiment, a thickness of oxide layer 203 isdetermined based on maintaining a parasitic capacitance between thedevice and the substrate layer at an acceptable level, such as less than1 pf, for example. Substrate layer 204 is deep etched to form a trench206 which, as will be described in greater detail below, is configuredto receive indenter tip 205. According to one embodiment, a thickness ofsubstrate layer 204 is selected as necessary to contain indenter tip 205as well as several tens of microns of an epoxy layer (not shown).According to one embodiment, indenter tip 205 comprises a diamond tip,for example. According to one embodiment, deep trench 206 ismicromachined on substrate layer 204.

FIGS. 4 and 5 below illustrate embodiments of actuation capacitor 120 ingreater detail. FIG. 4 is a diagram generally illustrating two sets ofelectrostatic actuation comb capacitors 140 and 142 of actuationcapacitor 120, according to one embodiment. While actuation capacitor120 includes more than two sets of actuation comb capacitors (see FIG. 5below), for ease of illustration, only two sets of actuation combcapacitors (i.e. 140 and 142) are shown in FIG. 4.

Electrostatic actuation comb capacitors 140 and 142 respectively includefixed electrode combs 144 and 146 extending from body 102 and movableelectrode combs 148 and 150 extending from a lateral edge of andmoveable with moveable probe 104. According to one embodiment, a smallgap, as illustrated by gap 152 between fixed electrode comb 144 andmovable electrode comb 148 has a gap distance three times smaller thanlarger gap 154 between movable electrode comb 148 and fixed electrodecomb 146. According to one embodiment, when fixed electrode comb 144 andmoveable electrode comb 148 are biased, an electrostatic force insmaller gap 152 becomes 9 times greater than that in larger gap 154creating a differential force which pulls movable probe 104 inindentation direction 116.

In FIG. 4, an overlapping width, b, between the fixed and moveableelectrode combs is illustrated at 156. Additionally, a section A-Athrough electrostatic actuation capacitor 142 illustrates, as indicatedat 158, an overlapping height, h, between fixed and moveable electrodecombs 146 and 150.

It is noted that actuation comb capacitors 140 and 142 are illustratedin their “home” or “zero” positions when actuation comb capacitors 140and 142 are unbiased and MEMS nanoindenter transducer 100 is notengaging a test sample. As such, according to one embodiment, asillustrated by FIG. 4, large gap 154 has a gap distance which is threetimes greater than small gap 152 (i.e. the moveable electrodes are notdisposed at equal distances between fixed electrodes).

As mentioned above, to actuate or displace moveable probe 104 andindenter tip 205 in indentation direction 116, a bias voltage is appliedto the electrostatic actuation comb capacitors of actuation capacitor120 to generate an electrostatic force between the fixed and moveableelectrodes, such as between fixed and moveable electrode combs 144 and148. The electrostatic force displaces moveable probe 104 in indentationdirection 116 against a countering force from springs 106, 108, 110, and112 which attempt to maintain moveable probe 104 in the so-called homeposition. According to one embodiment, a bias voltage is applied tofixed electrodes combs, such as fixed electrode combs 144 and 146, whilethe corresponding moveable electrode combs, such as moveable electrodecombs 148 and 150 are at a fixed voltage relative to the bias voltage,such as at ground, for example.

Actuation capacitor 120 employs an electrostatic force generated by achange in capacitance of each set of electrostatic actuation combcapacitors (e.g. electrostatic actuation comb capacitors 140 and 142 ofFIG. 4) resulting from an applied bias voltage. The capacitance ofactuation capacitor 120 can be changed by changing a gap between thefixed and moveable electrode combs or by changing an overlapping area ofthe fixed and moveable electrode combs (e.g. fixed and moveableelectrode combs 144 and 148 of FIG. 4). For a gap changing operation, anelectrostatic force generated between two electrode combs, such as fixedand moveable electrode combs 144 and 148, can be represented by EquationI as follows:

${F_{d} = {\frac{ɛ\; {bh}}{2d^{2}}V^{2}}};$

where the F_(d) is the electrostatic force to the gap changingdirection, ε is the dielectric permittivity, b represents an overlappingwidth of the electrodes (see FIG. 4), h is an overlapping height of theelectrodes (see FIG. 4), d is the gap between electrodes (see FIG. 4),and V is the applied or bias voltage.

Although comb drive actuators can generate a larger force by making alarge capacitance change with respect to the gap change, a comb driveoperated with a gap changing scheme has a travel range which isrelatively small due to the limited gap between electrode combs.Conversely, an overlapping area change scheme may have a large travelrange since travel is not limited by an electrode gap, but does notprovide as large a force as compared to a gap closing actuation scheme.

It is noted that some MEMS-based nanomechanical testers have actuationcapabilities (see References 8, 9, 10). Some such MEMS mechanicaltesters (see References 9 and 10) use overlapping area change as anactuation scheme, and another (see Reference 8) uses a gap closingscheme to generate the force. Among the two operation schemes, the gapclosing scheme is suitable for nanoindentation applications because,such applications do not require a large travel range (e.g. a 1 μmdisplacement), but do require a large indentation force (e.g. up to 1mN). As such, according to one embodiment, MEMS nanoindenter transducer100 employs a gap closing scheme as described above.

FIG. 5 is a microscope image of portions of a fabricated MEMSnanoindenter transducer 100, according to one embodiment, illustratingactuation capacitor 120. In the image of FIG. 5, actuation capacitor 120includes forty-eight sets of electrostatic actuation comb capacitors,with twenty-four being positioned on each of the opposite lateral sidesof moveable probe 104. Springs 106 and 108, and crash protectors 160 and162 are also visible in the image of FIG. 5.

FIGS. 6 and 7 below illustrate embodiments of sensing capacitors 130,132, 134, and 136 in greater detail. FIG. 6 is a diagram schematicallyillustrating the configuration and operation of sensing capacitors 130,132, 134, and 136 of micromachined comb drive 119 (see FIG. 2),according to one embodiment. Similar to electrostatic actuation combcapacitors 140 and 142 of actuation capacitor 120 as illustrated aboveby FIG. 4, sensing capacitors 130, 132, 134, and 136 each include aplurality of sets of fixed and moveable electrode combs.

For ease of illustration, each of the sensing capacitors 130, 132, 134,and 136 is shown in FIG. 6 as having only 3 sets of comb capacitors,with each set having a fixed electrode coupled to body 102 and amoveable electrode coupled to and displaceable together with moveableprobe 104. In other embodiments, each of the sensing capacitors 130,132, 134, and 136 may include more or less than 3 sets of combcapacitors (see FIG. 7 below). It is noted that a moveable electrode 180is shared by sensing capacitors 130 and 132, and that a moveableelectrode 182 is shared by sensing capacitors 134 and 136.

According to one embodiment, MEMS nanoindenter transducer 100 employs adifferential capacitive sensing scheme to detect and measuredisplacement of movable probe 104. When moveable probe 104 is displaced,such as from application of a bias voltage to the fixed electrode combsof actuation capacitor 120, gaps between the fixed electrode combs andthe moveable electrode combs of each of the sensing capacitors 130, 132,134, and 136 change which, in turn, changes the capacitance of each ofthe sensing capacitors 130, 132, 134, and 136.

The combined capacitance of all sets of comb capacitors for each of thesensing capacitors 130, 132, 134, and 138 in FIG. 6 are respectivelyrepresented as C_(A), C_(B), C_(C), and C_(D). It is noted thatcapacitance values C_(A), C_(B), C_(C), and C_(D) represent capacitivesignals 50 provided to detection circuit 60, as illustrated by FIG. 1.Based on changes in the values of capacitances C_(A), C_(B), C_(C), andC_(D) relative to known reference values for these capacitances whenmoveable probe 104 is an unbiased state and not engaging a test sample(i.e. moveable probe is at a “home” position), the displacement ofmoveable electrode 104 in the indentation direction 116 (i.e. z-axis),in the lateral direction (x-axis), and rotation of moveable electrode104 about the y-axis can be determined.

Displacement of moveable electrode 104 in indentation direction 116 isdetermined based on a capacitance combination ratio (CCR_(I)) expressedby Equation II as follows:

CCR_(I)={(C _(A) +C _(D))−(C _(B) +C _(C))}/{(C _(A) +C _(D))+(C _(B) +C_(C))}.

When moveable electrode 104 is moved in indentation direction 116, thesum of (C_(A)+C_(D)) increases while the sum of (C_(B)+C_(C)) decreases,resulting in an increase in {(C_(A)+C_(D))−(C_(B)+C_(C))}. Consequently,the value of CCR_(I) increases relative to a reference value forCCR_(I), determined using the known reference values for C_(A), C_(B),C_(C), and C_(D), by an amount that is proportional to the displacementof moveable probe 104 in indentation direction 116 (i.e. z-axis).

Displacement of moveable electrode 104 in the lateral direction (i.e.along the x-axis) is determined based on a capacitive combination ratio(CCR_(L)) expressed by Equation III as follows:

CCR_(L)={(C _(A) +C _(B))−(C _(C) +C _(D))}/{(C _(A) +C _(B))+(C _(C) +C_(D))}.

When moveable probe 104 moves in the lateral direction (i.e. x-axis) themoveable electrode combs of sensing capacitors 130, 132, 134, and 136move laterally relative to the fixed electrode combs so that the sum of(C_(A)+C_(B)) increases while the sum of (C_(C)+C_(D)) decreases due toa change in the overlapping area of the fixed and moveable electrodecombs, resulting in an increase in {(C_(A)+C_(B))−(C_(C)+C_(D))}.Consequently, the value of CCR_(L) increases relative to a referencevalue for CCR_(L), determined using the known reference values forC_(A), C_(B), C_(C), and C_(D), by an amount that is proportional to thedisplacement of moveable probe 104 in the lateral direction (i.e.x-axis).

Rotation movement of moveable electrode 104 about the y-axis, asindicated at 184, is determined based on a capacitive combination ratio(CCRR) expressed by Equation IV as follows:

CCR_(R)={(C _(B) +C _(D))−(C _(A) +C _(C))}/{(C _(B) +C _(D))+(C _(A) +C_(C))}.

When moveable probe 104 rotates in a clockwise direction, the sum of(C_(B)+C_(D)) increases while the sum of (C_(A)+C_(C)) decreases due tothe rotational motion, resulting in an increase in{(C_(B)+C_(D))−(C_(A)+C_(C))}. Consequently, the value of CCR_(R)increases relative to a reference value for CCR_(R), determined usingthe known reference values for C_(A), C_(B), C_(C), and C_(D), by anamount that is proportional to the angular rotation of moveable probe104.

Unlike a two-electrode capacitive sensor, the differential capacitivesensor as described above provides a more accurate displacementmeasurement regardless of environment changes such as temperature andhumidity variations. This provides great advantage of utilizing thedifferential sensing scheme for the applications in nano-scalemeasurement in a variety of environmental conditions.

FIG. 7 is a microscope image of portions of a fabricated MEMSnanoindenter transducer, according to one embodiment, illustratingsensing capacitors 130, 132, 134, and 136. In the image of FIG. 7, eachof the sensing capacitors 130, 132, 134, and 136 includes eighteen setsof comb capacitors disposed along lateral edges of moveable probe 104.Springs 110 and 112, and crash protectors 164 and 166 are also visiblein the image of FIG. 7.

FIG. 8 is a diagram generally illustrating a spring, according to oneembodiment, such as spring 106 of FIGS. 2 and 5. According to oneembodiment, as illustrated by spring 106, each of the springs 106, 108,110, and 112 have a greater stiffness to displacement in the lateraldirections (x-axis and y-axis) as compared to a stiffness todisplacement in indentation direction 116 (z-axis). Accordingly, eachspring has thin, long segments 190 in the lateral direction along thex-axis, and a thick, short segment 192 in indentation direction 116.Such a spring design substantially limits dislocation of indenter tip205 of moveable probe 104 from displacement axis 114 in the x and ydirections which might otherwise occur due to friction during anindentation procedure. Such spring characteristic is important forin-situ TEM nano-indentation especially when a sample surface is notperpendicular to the indentation direction.

Static characteristics of MEMS nanoindenter transducer 100 wereevaluated using finite element analysis. Stress distribution of springs106, 108, 110, and 112 was evaluated with moveable probe 104 having a1-μm displacement in indentation direction 116 (i.e. along the z-axis).According to the evaluation, a maximum stress of 75.2 MPa wasdetermined, which is far less than the yield strength of single crystalsilicon which is 7 GPa (see Reference 20). Such a large differencebetween the maximum stress and the yield strength indicates that a 1-μmdisplacement of moveable probe 104 is safe and would not result in anyplastic deformation or permanent damage of springs 106, 108, 110, and112. This low stress also enables springs 106, 108, 110, and 112 to keeplinear elastic behavior within the operational range of MEMSnanoindenter transducer 100.

FIG. 9 is a microscope image of portions of a fabricated MEMSnanoindenter transducer, according to one embodiment, illustrating crashprotector 166. As noted above, crash protectors 160, 162, 164, and 166prevent damage to electronics and to the fixed and moveable electrodecombs of actuation capacitor 120 and sensing capacitors 130, 132, 134,and 136 which might otherwise occur from contact between the fixed andmoveable electrode combs due to misoperation or mishandling. Asmentioned above, according to one embodiment, crash protectors 160, 162,164, and 166 are fabricated in device layer 202 (see FIG. 3).

According to one embodiment, as illustrated by crash protector 166 ofFIG. 9, a gap 190 is formed along the z-axis (i.e. in the direction ofthe displacement axis 114, see FIG. 2) between body 102 and moveableprobe 104, and a gap 192 is formed along the x-axis (lateral direction)between body 102 and moveable probe 104. According to one embodiment,gaps 190 and 192 have a gap distance of 5 μm so as to limit thedisplacement of moveable probe 104 along the z-axis (including in theindentation direction 116) and the x-axis to 5 μm. This 5 μmdisplacement limit is less than a gap distance between the fixed andmoveable electrode combs of actuation capacitor 120 and sensingcapacitors 130, 132, 134, and 136 (e.g. gap 152 as shown in FIG. 4)which, according to one embodiment is 10 μm. In addition, contact of thecrash protectors 160, 162, 164, and 166 with corresponding portions ofbody 102 is not electrically catastrophic since moveable probe 104 andsaid corresponding portions of body 102 are at a same potential (e.g.ground). According to tests performed on such an embodiment, crashprotectors 160, 162, 164, and 166 functioned properly and preventeddamage after multiple “pull-in” operations where large displacements ofmoveable probe 104 were performed.

FIG. 10 illustrates a measured amplitude response 220 and a measuredphase response 222 of MEMS nanoindenter transducer 100, according to oneembodiment, around a resonance frequency according to a frequencyresponse test of MEMS nanoindenter transducer 100 as measured with alock-in amplifier. In the illustrated example of FIG. 10, the resonancefrequency of the measured frequency response is 3.55 kHz. This highresonance frequency indicates a high bandwidth characteristic for thedynamics of MEMS microindenter transducer 100. This high bandwidthcharacteristic provides superior dynamic characteristic in nano-indenteroperation. In general, quality operation of a MEMS transducer is basedon precision motion control of the moveable probe. A high bandwidthcharacteristic helps increase the operational speed in the open loopcontrol system and reduces tracking error in a closed loop controlsystem. Improving the closed loop control performance is beneficial toidentifying sudden discontinuous changes in the nanoindentation data inorder to identify and investigate dislocation generation duringnanoindentation (see Reference 21).

In addition, a high bandwidth characteristic benefits the investigationof the dynamic characteristics of a sample at a higher frequency rangein a dynamic mechanical analysis (DMA) operation (see Reference 22).Furthermore, a high bandwidth characteristic enables an increasedscanning rate in topography imaging and modulus mapping (see Reference23) with no loss of image quality. According to one embodiment, the MEMSnanoindenter transducer 100 has 15 times higher bandwidth compared to aknown conventional transducer (see Reference 24) and it is capable of 15times faster imaging when integrated with a high bandwidth scanner.

According to one embodiment, a mechanical quality factor estimated fromthe frequency response is 320. Such a low damping characteristictogether with a high mechanical quality factor provides clear contrastin modulus mapping, especially for soft samples which need high forcesensitivity. In general, when a transducer is excited near the resonancefrequency, amplitude reduction to the reaction from the test sample isinversely proportional to the mechanical quality factor. As such, atransducer with a larger mechanical quality factor, such as MEMSnanoindenter transducer 100, has higher force sensitivity.

FIG. 11 is a diagram illustrating a first mode shape 230 of moveableelectrode 104 of MEMS nanoindenter transducer 100 at the resonancefrequency obtained from performance of the finite element analysis.Moveable probe 104 oscillates along indentation direction 116, whichverifies that such a mode can be employed for the dynamic mechanicalanalysis (DMA) testing.

An estimated second natural frequency is 16 kHz, and there is a largediscrepancy between the first and the second natural frequencies. Such alarge discrepancy completely decouples the first and the second modes indynamic operation and enables a better result with DMA testing whichutilizes the amplitude and phase responses. This DMA analysis is basedon a single-degree-of-freedom assumption and, to hold the assumption,complete separation of the second mode from the first mode is required.When the second mode is coupled with the first mode, the frequencyresponse around the resonance does not match with thesingle-degree-of-freedom second-order-system response and results inerrors in DMA testing; this needs to be considered when designing anindenter transducer.

FIG. 12 is a microscope image of portions of MEMS nanoindentertransducer 100 illustrating a bottom or backside of moveable probe 104and shows indenter tip mounting trench 206 fabricated in substrate layer204. The deep and long trench 206 on the backside of the substrateenables the mounting an indenter tip, such as indenter tip 205, withoutdamaging MEMS nanoindenter transducer 100. The long and narrowcharacteristics of mounting trench 206 help to align indenter tip 205with the desired direction (i.e. indentation direction 116). An openside of mounting trench 206 enables epoxy to be applied after themounting of indenter tip 205. According to one embodiment, indenter tip205 is attached in mounting trench 206 using an epoxy. According to oneembodiment, the indenter tip 205 is attached in mounting trench 206using an electrically conductive epoxy.

A contact area between indenter tip 205 and moveable electrode 104 iselectrically isolated from other portions of MEMS nanoindentertransducer 100, including actuation capacitor 120 and sensing capacitors130, 132, 134, and 136. Such electrical isolation enables MEMSnanoindenter transducer 100 to be used for applications in electricalmeasurement and electron microscopy in-situ testing. Electricalmeasurement during nanoindentation provides correlation between theelectrical measurement change and nanoindentation. Electrically isolatedconductive indenter tip 205 can also be used to discharge electrons forin-situ electron microscopy tests.

An electron charged indenter tip causes large attractive force andresults in jump-to-contact (see Reference 4). This attraction by theaccumulated electrons is undesirable because it distorts themeasurements data by adding the attraction to the indentationloading/unloading curve. Therefore, discharging the electrons bygrounding the electrically isolated conductive tip improves theperformance of MEMS nanoindenter transducer 100 for applications inin-situ electron microscopy testing.

FIG. 13 is a process flow diagram generally illustrating one embodimentof a process 300 of fabrication of MEMS nanoindenter transducer 100using silicon micromachining techniques. Process 300 begins at 302 witha starting material. According to one embodiment, the starting materialcomprises a silicon-on-insulator (SOI) wafer. According to oneembodiment, as described above, heavily boron doped p-type siliconwafers were used for device and substrate layers 202 and 204 in order toachieve a high electrical conductivity. According to one embodiment, aresistivity of the wafer ranges from 0.005-0.02 ohm-cm.

At 304, an oxide is deposited on the rear or back side of substratelayer 204. At 306, the oxide deposited at 304 is opened, such as viareactive ion etching (RIE), using a mask (e.g. photoresist) having apattern including the desired shape and dimensions of moveable probe104.

At 308, metal is deposited on device layer 202, followed at 310 byformation of a mask having a desired pattern and etching of device layer202 via deep reactive ion etching (DRIE). At 312, substrate layer 204 isetched (e.g. DRIE) via the patterned oxide on the back side thereof. At314, the oxide layer deposited at 204 is removed and insulator layer 203is etched via previously etched substrate layer 204.

FIG. 14 is a microscope image of a MEMS nanoindenter transduceraccording to the present embodiments, such as MEMS nanoindentertransducer 100, after fabrication.

MEMS nanoindenter transducer 100 includes many small features which arevulnerable to contamination. Such contamination may be prevented byproper packaging. According to one embodiment, for packaging purposes,top and bottom covers were micromachined to enclose MEMS nanoindentertransducer 100 of FIG. 14.

FIGS. 15 and 16 are microscope images respectively illustrating a topcover 401 and a bottom cover 403. Top and bottom covers 401 and 403respectively include trenches 402 and 404 which are configured toreceive MEMS nanoindenter transducer 100 when mounted thereto, leavingmoveable probe 104 and indenter tip 205 free to move in indentationdirection 116. After mounting to MEMS nanoindenter transducer 100, topand bottom covers 401 and 403 prevent physical contact with the movableprobe 104 and actuation and sensing capacitor 120, 130, 132, 134, and136. According to one embodiment, top and bottom covers 401 and 403 arefabricated from low-resistivity silicon and, in addition to physicalprotection, provide electrical shielding to MEMS nanoindenter transducer100.

FIG. 17 is an image of an example of a packaged MEMS nanoindentertransducer 410 after packaging MEMS nanoindenter transducer 100 with topand bottom covers 401 and 403. According to one embodiment, the overallsize of packaged MEMS nanoindenter transducer 410 was measured to be 2.8mm×0.98 mm×5.7 mm, including an epoxy layer employed to bond top andbottom covers 401 and 403 to MEMS nanoindenter transducer 100. Accordingto one embodiment, packaged MEMS nanoindenter transducer 410 iselectrically connected to a readout circuit using wire bondingtechniques. Wire bonding eliminates uncertainty in electricalinterconnection as all the bonding electrodes are able to be microscopeinspected. As the result, the wire-bonded transducer packages 410 showedexcellent electrical interface.

In one embodiment, the comb drive nanoindenter was integrated with aTriboIndenter® from Hysitron, Inc. (see Reference 25) and indentationand topography imaging was performed. Owing to its excellentcompatibility with existing Hysitron controllers and software, this testcould be done without instrument modification.

FIGS. 18 and 19 respectively illustrate load-displacement curves 501 and601 along with corresponding indent topography images 502 and 602obtained from the indentation experiments on a polycarbonate sample anda gold sample, such as via imaging device 70 (see FIG. 1). A Berkovichdiamond tip was used for indenter tip 205 and an open loop trapezoidload function with 5-second loading, 2-second peak force maintenance,and 5-second unloading was used for both indentation experiments. Theelastic and plastic deformations during the indentations were clearlyshown in the loading/unloading curves. Loading/unloading curves 501 and601 indicate that a MEMS nanoindenter transducer 100, according to thepresent embodiments, has indentation capability with high precisionforce control and high resolution displacement sensing.

Topography images 502 and 602 show the scanning capability of MEMSnanoindenter transducer 100. The images were taken at 3-Hz line scanrate which is the

TriboIndenter system's maximum scanning rate. The high quality imagetaken at high speed scanning is ascribed to the high bandwidth dynamiccharacteristic of MEMS nanoindenter transducer 100. In addition to widebandwidth, MEMS nanoindenter transducer 100 has a large lateralstiffness (10 times larger than indentation direction) and provides highimage quality by reducing negative effects from lateral friction.

To increase the scanning speed at the maximum line scanning rate 3-Hz, alarge area was scanned. FIG. 20 includes images 700 and 702 whichrespectively illustrate 5 μm×5 μm and a 40 μm×40 μm scanned topographyimages on a PMMA sample. Two adjacent cavities are clear in both scannedimages 700 and 702. The increase in scanning area also increases thescanning speed. The image quality at the higher speed scanning is notdegraded with a high bandwidth transducer, such as MEMS nanoindentertransducer 100.

Modulus mapping is a technique used to investigate the properties of amaterial within a specific area, such as storage modulus and lossmodulus, for example. According to one embodiment, for modulus mapping,the indenter is excited at a specific frequency and the amplitude andphase responses are measured by a lock-in amplifier. Modulus mappinguses a DC force as a control feedback and records the topography,amplitude and phase data while scanning the specified area. Themechanical properties of a sample are estimated from the measuredamplitude and phase data. The modulus mapping capability of MEMSnanoindenter transducer 100 was investigated by performing a dynamicindentation on a ceramic fiber sample.

FIG. 21 includes a topography image 800, an amplitude image 802, and aphase image 804 illustrating modulus mapping related images of a 30μm×30 μm scan area. For this example, the indenter was excited at 200 Hzand the sample was mapped with the line scan rate of 0.2 Hz. Anoperational setup with 10 μN of DC and AC forces and 1 ms time constantwas used for this modulus mapping experiment. Topography image 800,amplitude image 802, and phase image 804 were record simultaneouslyduring the mapping experiment. Amplitude and phase images 802 and 804show clear contrast between the two different materials having differentmechanical properties. Using this information and the tip shape, we canconvert the data to mechanical properties such as storage modulus andloss modulus, for example.

In summary, a micromachined MEMS nanoindenter transducer employing amicromachined comb drive is described, such as MEMS nanoindentertransducer 100 employing micromachined comb drive 119 (see FIG. 2). TheMEMS nanoindenter transducer described by the present disclosure can beused in electron microscopy as well as ambient conditions. All therequirements as a nanoindenter and also in-situ TEM nano-mechanicaltester were considered through the design and fabrication and thedeveloped MEMS nanoindenter transducer satisfies required specificationssuch as physical dimensions, maximum force, spring stiffness, forcesensitivity, dynamic bandwidth, travel range, and material restrictions.Experimental results with the MEMS nanoindenter transducer andHysitron's instruments showed excellent instrument compatibility andversatile mechanical testing capabilities. Indentation, topographyscanning, and dynamic testing capabilities were proven from therepeatable and robust nanoindenter operations. The MEMS nanoindentertransducer 100 can also be physically integrated into a variety of TEMholders and expands quantitative in-situ TEM nano-mechanical testingapplication to various TEMs which has been hindered by large transducersize. It is noted that a MEMS nanoindenter transducer according to thepresent disclosure, such as MEMS nanoindenter transducer 100, can alsobe incorporated into an SEM (scanning electron microscope) for in-situmechanical testing applications.

In addition to these applications, a MEMS nanoindenter transduceraccording to the present disclosure can be applied to a variety ofapplications by integration into various instruments. For example, withits high bandwidth dynamic characteristic, the MEMS nanoindentertransducer can be used for high speed imaging and high speed modulusmapping. The high bandwidth characteristic also provides high frequencyDMA testing capability. The low damping characteristic with highmechanical quality factor makes the dynamic responses sensitive to thesample interaction when the MEMS nanoindenter transducer is operatednear the resonance frequency and can be used for topography measurementwithout damaging the sample surface. This is especially advantageous toincrease the accuracy in measuring the indent on soft samples.

Another possible application is in-situ electrical measurement. Theseparated electrode line for the tip can be used to measure theelectrical characteristic while doing indentation. In addition to theapplications in quantitative in-situ mechanical testing, by utilizingits small size, the MEMS nanoindenter transducer can be integrated withvarious precision instruments, such as miniature manipulators, and cando mechanical property inspections and surface modifications in a smallspace.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

1. A microelectromechanical (MEMS) nanoindenter transducer comprising: abody; a probe moveable relative to the body; an indenter tip coupled toan end of the moveable probe, the indenter tip moveable together withthe probe; and a micromachined comb drive including: an electrostaticactuator capacitor comprising a plurality of comb capacitors configuredto drive the probe, together with the indenter tip, along a displacementaxis, including in an indentation direction, upon application of a biasvoltage to the actuation capacitor; and a plurality of sensingcapacitors forming a differential capacitive displacement sensor, eachsensing capacitor comprising a plurality of comb capacitors and eachconfigured to provide capacitance levels which, together, arerepresentative of a position of the probe, wherein each of the combcapacitors of the actuator capacitor and the sensing capacitors includesa fixed electrode comb coupled to the body and a moveable electrode combcoupled to the probe.
 2. The MEMS nanoindenter transducer of claim 1,wherein the probe is coupled to the body by a plurality of deflectablesprings.
 3. The MEMS nanoindenter transducer of claim 2, wherein thesprings maintain the probe at a home position when the actuatorcapacitor is in an unbiased condition and the indenter tip is disengagedfrom an external test surface.
 4. The MEMS nanoindenter transducer ofclaim 2, wherein the springs provide a stiffness against movement alongthe displacement axis which is less than a stiffness against movementalong axes which are perpendicular to the displacement axis.
 5. The MEMSnanoindenter transducer of claim 1, wherein the MEMS nanoindentertransducer has a bandwidth in a range from 1 kHz to 5 kHz.
 6. The MEMSnanoindenter transducer of claim 1, wherein the indenter tip iselectrically isolated from the micromachined comb drive.
 7. The MEMSnanoindenter transducer of claim 1, wherein the end of the probeincludes a trench in which the indenter tip is mounted, the trenchproviding alignment of the indenter tip along the displacement axis. 8.The MEMS nanoindenter transducer of claim 1, wherein the indenter tip isattached to the probe using an electrically conductive epoxy.
 9. TheMEMS nanoindenter transducer of claim 1, wherein the indenter tipcomprises conductive diamond.
 10. The MEMS nanoindenter transducer ofclaim 1, including crash protectors which limit movement of the probe ineach direction along the displacement axis to a distance less than a gapbetween fixed and moveable electrode combs of the comb capacitors of theactuator and sensing capacitors.
 11. The MEMS nanoindenter transducer ofclaim 10, where the crash protectors limit movement of the probe in thedisplacement direction to not more than 5 micrometers.
 12. The MEMSnanoindenter transducer of claim 1, wherein the plurality of sensingcapacitors comprises four sensing capacitors with two pairs of sensingcapacitors mounted opposite one another along longitudinal edges of theprobe which are parallel to the displacement axis.
 13. The MEMSnanoindenter transducer of claim 12, wherein the four sensing capacitorstogether provide capacitance levels representative of lineardisplacement of the probe along the displacement axis.
 14. The MEMSnanoindenter transducer of claim 12, where the four sensing capacitorstogether provide capacitance levels indicative of linear displacement ofthe probe along an axis perpendicular to the displacement axis.
 15. TheMEMS nanoindenter transducer of claim 12, wherein the four sensingcapacitors together provide capacitance levels indicative of rotationaldisplacement about an axis perpendicular to the displacement axis. 16.The MEMS nanoindenter of claim 1, wherein the body, probe, andmicromachined comb drive are housed within a cover fabricated from alow-resistivity material, wherein the cover provides protection fromcontamination and provides electrical shielding to reduce parasiticcapacitance and noise.
 17. A nanoindentation test system comprising: aholder for retaining a sample; a microelectromechanical (MEMS)nanoindenter transducer including: a body; a probe moveable relative tothe body; an indenter tip coupled an end of the moveable probe; and amicromachined comb drive including: an actuator capacitor comprising aplurality of comb capacitors configured to drive the probe, togetherwith the indenter tip, along a displacement axis upon application of abias voltage to the actuation capacitor; and a plurality of sensingcapacitors forming a differential capacitive displacement sensor, eachsensing capacitor comprising a plurality of comb capacitors and eachconfigured to provide capacitance levels which, together, arerepresentative of a position of the probe, wherein each of the combcapacitors of the actuator capacitor and the sensing capacitors includesa fixed electrode comb coupled to the body and a moveable electrode combcoupled to the probe; and a controller configured drive the probe alongthe displacement axis so as to provide a desired force to the samplewith the indenter tip by applying a desired bias voltage to theactuation capacitor.
 18. The nanoindentation test system of claim 17,wherein the controller is configured to determine a displacement of theprobe in response to application of the bias voltage having the desiredlevel based on capacitance levels of the sensing capacitors.
 19. Thenanoindentation test system of claim 18, wherein the plurality ofsensing capacitors comprises four sensing capacitors, and wherein thecontroller, from the capacitance levels provided by the four sensingcapacitors, is configured to determine a linear displacement of theprobe along the displacement axis, a linear displacement of the probealong an axis perpendicular to the displacement axis and in a directionof a width of the body, and rotational displacement about an axisperpendicular to the displacement axis in and in a direction of athickness of the body.
 20. The nanoindentation test system of claim 17,wherein the indenter tip is electrically isolated from comb driveelectrode combs.
 21. The nanoindentation test system of claim 17,wherein the nanoindentation test system comprises a quantitativeelectron microscopy (TEM) in-situ mechanical test system.
 22. A methodof nanoindentation testing for determining material properties of sampleemploying a microelectromechanical (MEMS) nanoindenter transducer, themethod comprising: applying a desired bias voltage to an actuatorcapacitor of a micromachined comb drive to drive a moveable probe alonga displacement axis in an indentation direction toward the sample sothat an indenter tip coupled to the probe contacts and provides adesired force to a surface of the sample, wherein the actuator capacitorcomprises a plurality of comb capacitors, each comb capacitor comprisinga moveable electrode comb coupled to the probe and a fixed electrodecomb; and determining a displacement of the indenter tip by measuringthe capacitance of a plurality of sensing capacitors of themicromachined comb drive, the sensing capacitors together forming adifferential capacitive displacement sensor, each sensing capacitorcomprising a plurality of comb capacitors, each comb capacitorcomprising a moveable electrode comb coupled to the probe and a fixedelectrode comb.
 23. The method of claim 22, wherein measuring thedisplacement of the indenter tip includes determining a lineardisplacement of the probe along the displacement axis, a lineardisplacement of the probe along a first axis perpendicular to thedisplacement axis, and a rotational displacement of the probe aboutsecond axis perpendicular to the displacement axis and to the firstaxis.